A wire saw is an apparatus in which a workpiece is pressed against a wire row spirally wound between a plurality of wire guides (grooved rollers), an abrasive fluid in a slurry state obtained by mixing free-abrasive grains into an oily or an aqueous coolant is supplied to a contact portion between the workpiece and a wire, and thereby the workpiece is sliced into many wafers by a grinding effect of the free-abrasive grains. Specifically, the free-abrasive grains in the supplied abrasive fluid are pushed into an inner portion of a wire groove (a slicing groove of the workpiece) by the wire to scrape a bottom portion of the groove of the workpiece while making the wire row travel in a reciprocating direction so that the workpiece is sliced.
As described above, the workpiece is sliced into wafers by scraping the free-abrasive grains on the inner portion of the wire groove (the slicing groove of the workpiece) by the wire traveling in a reciprocating direction. Friction that occurs in this case causes generation of heat at a slicing portion and thereby the workpiece is thermally expanded during slicing. Moreover, the wire guides are axially expanded by thermal expansion under frictional heat between the wire and the wire guides and under frictional heat generated at a bearing portion, which sustains the wire guides. A relative position of the workpiece with respect to the wire row, which is spirally wound between the wire guides, consequently changes during slicing.
A change in the relative position between the workpiece and the wire row, which is caused by the thermal expansion of the workpiece and the expansion in an axial direction of the wire guides, brings about inflecting slicing trajectory depicted in the workpiece by the wire. There arises a problem that this inflection of the slicing trajectory is detected as Warp in a shape measurement after wafer processing.
As a measure against the above-mentioned problem, with regard to the expansion in an axial direction of wire guides, Japanese Unexamined Patent publication (Kokai) No. 8-323741 discloses a method of controlling the expansion in an axis direction of main rollers (wire guides) by circulating coolant water in bearings of the main rollers (the wire guides). With regard to the expansion of a workpiece, Japanese Patent No. 3734018 discloses a method of controlling the temperature of a workplace by supplying a temperature control medium to the workplace.
According to the measures disclosed in the above patent documents, Warp of a sliced wafer is somewhat improved by suppressing the expansion in an axial direction of the wire guides and the thermal expansion of the workpiece and by preventing the relative position of the workpiece with respect to the wire row, which is spirally wound between the wire guides, from changing during slicing. However, an amount of the improvement is insufficient.